Interaction Between Magma & Altered Crust on a Mature Oceanic Island: An Isotope & Trace Element Study of the Diego Hernandez Fmtn, Las Canadas Caldera, Tenerife, Canary I

EAR-0001013Wolff/Larson我们对地幔如何工作的许多了解来自对大洋岛屿玄武岩中微量元素丰度和同位素比率的测量。两者都是对地幔来源和岩浆在地球表面喷发和冻结之前的运移历史的保守的“飞行记录器”。玄武岩地球化学在地幔中的应用依赖于玄武岩岩浆在穿过地壳到达地表的过程中不会受到显著影响的假设。虽然大多数科学家都承认，上升的岩浆可能在一定程度上同化了寄主岩石，但支持洋岛玄武岩作为源区化学忠实记录者的一个强有力的论点是，地壳寄主岩石--主要是较早喷发的玄武岩--对可能包含它们的岩浆没有化学杠杆作用。我们将通过研究一个极端案例来检验这一断言，并对海洋岛屿环境中合理预期的最大同化程度设定限制。与许多岛屿不同，特内里费岛在表面和岛屿结构内都广泛发育可熔的长英质岩石。此外，在我们准备研究的地层中，有大量证据表明地幔玄武岩岩浆与长英质岩浆密切混合，其中一些来自地下岩石的熔融。快速淬火辉岩中混合结构的保存将使我们能够详细研究玄武岩-长英质岩石的相互作用，而长英质岩石提供的成分杠杆将允许我们限制地壳相互作用对地幔同位素，特别是海洋-岛屿玄武岩的微量元素特征的影响程度。

EAR-0001013Wolff/LarsonMuch of our understanding of how Earth's mantle works comes from measurements of trace element abundances and isotope ratios in ocean island basalts. Both act as conservative "flight recorders" of the mantle sources and transport histories of magma up to the point of eruption and freezing upon Earth's surface. Application of basalt geochemistry to the mantle depends on the assumption that basaltic magmas are not significantly affected by their passage through the crust en route to the surface. While most scientists would admit the possibility of some degree of host-rock assimilation by rising magma, a powerful argument in favor of ocean island basalts as faithful recorders of source chemistry is that crustal host-rocks - dominantly earlier erupted basalts - exert no chemical leverage on the magma that they may be incorporated into. We shall test this assertion, and place limits on the maximum extent of assimilation that may reasonably be expected in an ocean island setting, by studying an extreme case. In contrast to many islands, Tenerife has an extensive development of fusible felsic rocks both on the surface and within the island structure. Also, in the formation we propose to study, there is abundant evidence for intimate mingling of mantle-derived basaltic magmas with felsic magma, some of which is derived from melting of subsurface rock. The preservation of mingling textures in rapidly-quenched ignimbrites will allow us to study basaltic-felsic interactions in detail, while the compositional leverage provided by the felsic rocks will allow us to constrain the extent to which the mantle isotopic and, especially, trace-element characteristics of ocean-island basalts are compromised by crustal interactions.